No, not all rats carry leptospirosis. Infection rates in wild rat populations vary dramatically by location, ranging from 0% in some areas to over 80% in others. In many urban environments, roughly 20% to 50% of wild rats are infected, making them a significant but not universal source of the disease.
How Many Wild Rats Actually Carry It
A large review of global studies found that leptospirosis prevalence in rats swings widely depending on geography and climate. In tropical countries like Brazil, India, and the Philippines, infection rates can exceed 80%. In cooler or drier climates, rates tend to be much lower. Studies in the United Kingdom found infection rates between 1% and 42% depending on the method used, while Canadian studies reported 0% to 18%.
Some specific findings give a clearer picture. In Hawaii, between 16% and 53% of rats tested positive. In New Zealand, roughly a quarter to a third carried the bacteria. Studies in Algeria found that 43% of captured rats showed evidence of past infection, and about 41% were actively shedding the bacteria. A handful of locations, including parts of Madagascar, Tanzania, and the Faroe Islands, reported zero prevalence in certain surveys.
Urban rats tend to carry higher rates than rural ones. In the Algeria study, rats captured in cities had significantly higher infection rates than those from rural areas, likely because dense populations and sewer systems create ideal conditions for the bacteria to spread between animals.
Brown Rats vs. Black Rats
The two most common rat species don’t carry leptospirosis at the same rate. Brown rats (the larger sewer rats found in most cities) are more likely to be infected than black rats (the smaller, more agile roof rats). A study in Israel found that 21% of brown rats carried the bacteria compared to about 10% of black rats. Brown rats are considered the single most important animal source of human leptospirosis infections worldwide.
Why Rats Spread It So Effectively
Rats are uniquely dangerous carriers because the bacteria doesn’t make them sick. After about three to four weeks of age, rats become essentially immune to developing any symptoms. Even when experimentally injected with high doses of the bacteria, adult rats show no weight loss, no illness, and no signs of major inflammation in their organs.
The bacteria colonizes the tiny tubes inside the rat’s kidneys and stays there permanently. An infected rat can shed up to 10 million bacterial cells per milliliter of urine, continuously, for months or even years. The bacteria achieves this by changing which proteins it displays on its surface while living inside the rat, effectively hiding from the animal’s immune system. The rat’s body produces antibodies, but those antibodies can’t recognize the altered version of the bacteria lodged in its kidneys. This makes rats lifelong shedders that contaminate their environment every time they urinate.
How the Bacteria Reaches Humans
People almost never catch leptospirosis directly from a rat. The far more common route is indirect: contact with water or soil that has been contaminated with infected rat urine. The bacteria enters the body through cuts, scrapes, or abrasions on the skin, or through mucous membranes like the eyes, mouth, or nose. Swallowing contaminated water while swimming is a well-documented risk factor.
The bacteria can survive in the environment for a surprisingly long time. In water-saturated soil, one strain persisted for up to 193 days. In tap water, bacteria have remained viable and capable of causing infection for eight months. Drier conditions kill them much faster, with survival dropping to as few as five days in merely damp soil. This is why leptospirosis outbreaks spike after flooding in tropical regions, when rat urine washes into standing water that people wade through.
Occupations that put people in contact with contaminated water or soil carry the highest risk. Sewer workers, farmers, and people who work around waterways are more likely to be exposed.
What About Pet Rats
Pet rats are not automatically safe. While they are far less likely to be infected than wild rats, documented cases of humans catching leptospirosis from pet rodents have been reported in both Belgium and France. In those cases, the disease was severe, and testing confirmed the bacteria was present in the animals’ kidneys and being shed in their urine.
The risk depends on where the pet came from and whether it has had any contact with wild rodents or contaminated environments. A pet rat bred in clean indoor conditions and kept away from wild animals carries very low risk. But the possibility is not zero, and owners who handle their animals frequently or clean cages without gloves do have a potential exposure route.